timer 555

[Fahime]

Hi there,
I'm building a timer for stairs. You can see the attached schematic.
The circuit is working well.But I'm meaning to make some changes in it for some reasons:
You know, the circuit is drawing current all the time even when nobody is using the light. I have seen the timers that one head of the switch in them is connected to the mains so whenever the switch is hitted the timer starts to work. But I can't figure out the trick for this circuit. Do you know the trick?
The other benefit of this circuit is that the number of the outputs reduce to 4 and so it's more commercial in wiring.
Appreciated for your assit.

 

[Gayan Soyza]

You mean when you step your foot onto the fist stair, the timer starts and turns on & after while its turning off like that?

I think you have already done that.

Monstable mode switch is the trick there’s no any other trick.

 

[ericgibbs]

hi fahime,

I see that you are switching the 'neutral' line with your relay contacts.

The problem is that the light socket and all the wiring from the mains supply
to the relay contact is 'live' all the time!

Ideally you should switch the 'live' side of the light.

There are circuits using the 555 that will give a delay/timing period when the power is applied to the 555. Simliar to what Gayan is suggesting.

A pressure pad placed under the stair carpet, at the foot or at the top of the stair makes a good trigger switch. You can buy pressure pads designed for stair tread sizes.

 

[bananasiong]

Is a inversed diode needed for the coil of the relay? Otherwise the ouput pin of the 555 timer may be destroyed, right?

 

[ericgibbs]

Is a inversed diode needed for the coil of the relay? Otherwise the ouput pin of the 555 timer may be destroyed, right?

Yes I agree!

 

[audioguru]

I have seen a 555 driving a relay coil with two protection diodes. The 2nd one is in series to prevent the clamped flyback voltage from exceeding the supply voltage.

 

[Hero999]

Which won't happen and the 0.7V across the diode isn't going to hurt.

 

[em2006]

A tipical protection for 555 driving relay (attach)

 

[bananasiong]

I have seen a 555 driving a relay coil with two protection diodes. The 2nd one is in series to prevent the clamped flyback voltage from exceeding the supply voltage.

Why the series diode is needed? I thought the reversed parallel diode limits the flyback voltage to 0.7 V? Is it needed for every relay switching design?

 

[audioguru]

The diode across the coil clamps the flyback voltage to be 0.7V higher than the supply voltage. I have seen many circuits with two diodes for a 555 that drives a relay.

I just looked through a few datasheets but I couldn't find a word anout it.

 

[Gayan Soyza]

Fahime sorry for changing your circuit.

But for the staircase to turn on & off a relay you don’t need a timer or an electronic circuit. Have you heard about two way switch method? Can do from it.But practically its hard to place the on off switch instead of push on switch.

Now I don’t have the diagram. If need I’ll draw & give.

But you have to step the last step also to turn off the relay.

 

[Fahime]

Your comments are very useful. Thanks everyone.
But I haven't gotten my answer yet.
In my design the switch is on the way of trigger signal. And it doesn't cut the mains when it's up. I wanna to put the switch on the way of the mains. What changes should be done in the circuit to fulfill this ?
If I'm not so clear please tell me to explain more.

 

[ericgibbs]

hi fahime,
Is this what you were asking???

 

[Gaston]

to keep the circuit from drawing current all of the time; run the hot lead going to the primary of the transformer through the relay also. then put the momentary switch in parallel with the relay contacts. then remove the momentary switch on the low voltage side. this will latch the transformer on until the 555 times out. then the whole thing will shut off

 

[em2006]

In my design the switch is on the way of trigger signal. And it doesn't cut the mains when it's up. I wanna to put the switch on the way of the mains. What changes should be done in the circuit to fulfill this ?

This is a solution.
(attach)

 

[Hero999]

The diode across the coil clamps the flyback voltage to be 0.7V higher than the supply voltage.

That's not true if the relay is connected to the 0V rail it will clamp it to -0.7V and if it's a CMOS 555 the diode will be bypassed when the n-mosfet turns on clamping the voltage even nearer 0V.

 

[jbelectric777]

Stairs lighting

Fahime,

What are you trying to accomplish ? Is this for stairs in a home ? Do you have three way switches at both the top and bottom of the stairs ? Because it seems a lot less complicated if you just controlled your relay via a set of S3's..... Besides, National and local electrical codes almost definetly require that wiring method in dwellings.

 

[jbelectric777]

the 555IC

fahime, looking at your schematic you have the relay directly connected to 0V and 9V to the other side from pin 3..... in the standby state the 555 pin 3 will be high (+9V) and pull in the relay , this may be what you mean when you say it's still drawing current. the "trick" you are talking about could be changing the polarity to the relay coil, in other words, from pin 3 connect one side of the relay and from the other side of the relay connect it direct to +9V, basically you will now be triggering when pin 3 goes low (0V) because in the other state you will have +9V on each side of the relay coil and it will not draw current until your switch triggers the IC, lemme know if thats what you were referring to.....Good Luck , I kinda had the same problem on another project until I realized pin 3 was high all the time (when not triggered) so I swapped the leads and it worked perfect.

 

[em2006]

An important advantage of the last posted schematic ( https://www.electro-tech-online.com/attachments/timer2-png.12957/ ):
There is no voltage on the electronics in stand-by. No need to swap the leads to the relay coil, it worked perfect.

 

[Fahime]

Dear Em2006 and Gaston,
That's it! Thanks a million.
But I'm not sure about the diod between coil and ground of relay.